On June 9, 1998, at 2110 central daylight time, a Cessna 172K, N758FY, sustained substantial damage when the left main gear collapsed during landing rollout on runway 35 (2180' x 40', turf, wet) at the North Omaha Airport, near Omaha, Nebraska. The private pilot and two passengers reported no injuries. The 14 Part 91 personal flight was operating in visual meteorological conditions at the time of the accident. There was no flight plan on file. The flight departed from Anita Municipal-Burke Memorial Field, near Anita, Iowa, at 2020. According to the pilot's written statement, the pilot stated that he made a normal approach to the runway, had a normal touchdown, and the aircraft bounced once during the landing rollout. Approximately 15 to 20 yards into the landing rollout the left axle broke, causing the left gear leg to penetrate into the turf runway. The left wing contacted the ground, aircraft nosed over, and then came to a stop in an inverted position. Pilot stated that he shut down all of the aircraft systems and evacuated the passengers from the aircraft. Post accident investigation showed no indications of a hard landing or significant side-loading applied to the landing gear. The aircraft had been converted into a conventional gear type aircraft under a Supplemental Type Certificate (STC) in June of 1995. Subsequent investigation done by an inspector with the Federal Aviation Administration (FAA) showed that the STC requires that there is an installation of wheel spindles inside the axle assemblies to provide reinforcement. On site investigation, completed by the FAA, indicated that the above noted spindle assemblies were not present at the time of the accident.